The present invention relates to a shift control apparatus for reducing gear change time without producing mechanical shock in the gear change operation of an automatic transmission in which two gear change mechanisms can change gears independently and are connected in series. The invention further relates to a control method therefor.
In an automatic transmission for a vehicle, a hydraulic pressure is selectively supplied to, and discharged from, frictional engaging elements such as clutches and brakes so that a gear change operation can be accomplished by some rotational elements of the automatic transmission. These rotational elements are connected to an input shaft of the transmission or fixed with respect to a casing of the gearbox in accordance with a drive condition of a vehicle.
If a shift control apparatus and frictional engaging elements are arranged along the shaft of the gearbox of the automatic transmission, particularly to that of the automatic transmission of more than four forward speeds, the size of the gearbox is elongated. If the longitudinal direction of a crank shaft is arranged perpendicular to the longitudinal direction of a car body, there are various practical problems.
Therefore, Japanese Patent Laid-Open Publication No. 59-113346 (1984) describes a compact automatic transmission in which two automatic gear change mechanisms capable of changing ratios of rotational speed independently are connected in series. One end of one gear change mechanism is connected to an output shaft of a fluid coupling and the other end is connected to an input shaft of another gear change mechanism. Further, the output shaft of the latter gear change mechanism is connected via an output shaft to a drive shaft.
The longitudinal length of the above described transmission can become shorter by arranging a first gear change mechanism and a second gear change mechanism in parallel. In this type of transmission, a first input shaft of the first gear change mechanism is coaxially connected to a crank shaft of a driving mechanism through a fluid coupling. A driving force is transmitted from a first output shaft of the first gear change mechanism to a second input shaft of the second gear change mechanism. The driving force is further transmitted to a drive shaft from the second output shaft of the second gear change mechanism. The first and second gear change mechanisms are thus provided in parallel. A first ratio of the rotational speed of the first output shaft connected to the second input shaft of the second gear change mechanism with respect to the first input shaft connected to the fluid coupling is changed in the first gear change mechanism. Further, a second ratio of the rotational speed of the second output shaft connected to the drive shaft with respect to the second input shaft connected to the first output shaft of the first gear change mechanism is simultaneously changed in the second gear change mechanism so that it is possible to shift a plurality of gear shifts, as desired. Plural frictional engaging elements are provided in the first gear change mechanism. Hydraulic oil is supplied to and discharged from the frictional engaging elements. The hydraulic oil is selectively supplied to the frictional engaging elements. Thereby the ratio of the rotational speed of the first output shaft of the first gear change mechanism is changed vis-a-vis the first input shaft. On the other hand, plural frictional engaging elements are also provided in the second gear change mechanism. The hydraulic oil is selectively supplied to those frictional engaging elements. Thereby the ratio of the rotational speed of the second output shaft of the second gear change mechanism is changed vis-a-vis the second input shaft.
In the automatic transmission in which two gear change mechanisms are arranged in parallel, shift shocks in the gear change operation independently occur in the respective gear change mechanisms. Therefore, it is designed not to change gear in the two gear change mechanisms at the same time. For example, if it is necessary to change a gear shift by operating the two gear change mechanisms in a conventional transmission, one gear change mechanism is operated and then the other gear change mechanism is operated in order to prevent a shift shock during the gear change operation.
If the gear is to be shifted to an adjacent gear ratio, one of two gear change mechanisms is operated. If the gear is to be shifted to a different gear ratio by skipping the adjacent gear ratio (hereinafter it is referred to as "skip operation"), one gear change mechanism is operated after an operation of the other gear change mechanism is completed. For example, if the gear is to be changed from the third forward speed to the fifth forward speed, the gear is shifted to the desired fifth forward speed after the gear is shifted to the fourth forward speed.
Therefore, in the above described conventional mechanism, there are practical problems. Although a skip operation is preferable, the gear is changed step by step. A period from the start to the end of the gear change operation becomes longer, a response characteristic with respect to the gear change becomes slow and a feeling of the gear change becomes dull.